The capacity of a lithium-ion battery may decrease over its lifetime due to a loss of the active material and/or consumption of charge via side reactions. Efforts have been made to decrease the loss of capacity via the discovery of improved materials.
Rechargeable lithium-ion batteries may be an attractive energy storage system for portable electronics and hybrid-electric vehicles because of their high energy density and rate capability. Such batteries, however, may experience degradation, which may limit their useful life. In particular, rechargeable lithium-ion batteries may experience a decrease in useable capacity (that is, “capacity fade”) and/or an increase in the internal resistance of the battery (that is, “power fade”). Here, the capacity fade may result from degradation or loss of the active material that serves as a host to the lithium ions in the working electrodes of the battery, or from loss of capacity due to side reactions at one or both of the working electrodes.
Other prior cells have been designed to compensate for first-cycle lithium loss during solid electrolyte interphase (SEI) formation, which itself may be a side reaction. In addition, U.S. Pat. No. 6,335,115, entitled “Secondary Lithium-ion Cell with an Auxiliary Electrode” (herein referred as “the Meissner reference”) discusses the use of an auxiliary lithium electrode that purportedly compensates for lithium loss throughout the life of the cell. In particular, the Meissner reference refers to ionic isolation and electronic isolation to isolate an auxiliary electrode from the working electrodes. According to the Meissner reference, ionic isolation involves an orientation of the battery in which the lithium-ion containing electrolyte contacts the two working electrodes, but not the auxiliary electrode. The auxiliary lithium electrode is presumably always in electronic contact with one of the working electrodes, but replenishment of lithium to the depleted working electrode does not occur until the cell is reoriented such that the electrolyte is in contact with both the working electrode and the auxiliary electrode.
The use of an auxiliary lithium electrode as discussed in the Meissner reference cannot be practically implemented in a lithium-ion battery because the battery design would require that the electrolyte not completely fill the pores of the separator and working electrodes. However, the porous separator could act as a wick to transport the electrolyte to the region of the separator that contacts the auxiliary electrode. Even residual electrolyte in the pores of this region of the separator would allow transport of lithium from the auxiliary electrode to the working electrode. Lithium transfer would continue until the potentials of the working and auxiliary electrodes equilibrated. Excessive lithium transfer beyond the point of capacity balance between the two working electrodes would result in reduction of the cell's capacity. (See Christensen et al., “Effect of anode film resistance on the charge/discharge capacity of a lithium-ion battery,” Journal of the Electrochemical Society, 150 (2003) A1416 (hereinafter referred to as “Christensen I”), and Christensen et al., “Cyclable Lithium and Capacity Loss in Li-ion Cells,” Journal of the Electrochemical Society, 152 (2005) A818 (hereinafter referred to as “Christensen II”)). Moreover, shorting of the auxiliary-electrode-working-electrode circuit via imperfect ionic isolation would lead to rapid transfer of lithium to the working electrode and possible deposition of lithium on the electrode surface. Such lithium deposition may pose a safety risk and/or degrade the cell because the lithium metal reacts rapidly and exothermically with the organic solvent used in the electrolyte. (See Aora et al., “Mathematical Modeling of the Lithium Deposition Overcharge Reaction in Lithium-ion Batteries Using Carbon-based Negative Electrodes,” Journal of the Electrochemical Society, 146 (1999) 3543).
Even if it were possible to maintain ionic isolation of the auxiliary electrode until lithium transfer is required, the cell design referred to by the Meissner reference would require additional electrode and separator material that is unutilized. Moreover, lithium transport between the two working electrodes of the cell would not be possible if the orientation of the cell were such that the two working electrodes are not in ionic contact. Indeed, even if the above-discussed concerns were addressed, relying upon reorientation of the battery significantly reduces the number of potential applications. For example, if battery-powered devices such as power tools are used in more than one orientation the auxiliary-electrode-working-electrode circuit could close unintentionally during the operation of the battery. Accordingly, the approach advocated by the Meissner reference is limited to applications having a fixed orientation.
In regards to electronic isolation, the Meissner reference requires that the lithium auxiliary electrode be placed between the positive and negative electrodes. However, such placement of the lithium auxiliary electrode would reduce the uniformity of the current distribution, and therefore the rate capability of the cell, when transferring lithium from one working electrode to the other. The Meissner reference may also require that the auxiliary electrode be connected to a metallic jacket.